N-(phosphonomethyl)glycine (known in the agricultural chemical industry as glyphosate) is described in Franz, U.S. Pat. No. 3,799,758. N-(phosphonomethyl)glycine and its salts are conveniently applied as a post-emergent herbicide in aqueous formulations. It is a highly effective and commercially important broad-spectrum herbicide useful in killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants.
Various methods for making N-(phosphonomethyl)glycine products are known in the art. One of the more widely accepted methods of making N-(phosphonomethyl)glycine compounds includes the liquid phase oxidative cleavage of a carboxymethyl substituent from an N-(phosphonomethyl)iminodiacetic acid substrate using an oxygen-containing gas in the presence of a heterogeneous oxidation catalyst. As used herein, “N-(phosphonomethyl)iminodiacetic acid substrates” include N-(phosphonomethyl)iminodiacetic acid (sometimes referred to as PMIDA) and salts thereof, wherein the salt-forming cation is, for example, ammonium, alkylammonium, an alkali metal or an alkaline earth metal. For example, N-(phosphonomethyl)glycine may be prepared by the liquid phase oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid with oxygen in accordance with the following reaction:

Other by-products also may form, such as formic acid, which is formed by the oxidation of the formaldehyde by-product, and aminomethylphosphonic acid (AMPA), which is formed by the oxidation of N-(phosphonomethyl)glycine. The preference for heterogeneous catalysis stems, at least in part, from the ease with which a particulate heterogeneous catalyst can normally be separated from the reaction product mixture for reuse following the oxidation. The literature is replete with examples of heterogeneous catalysis of N-(phosphonomethyl)iminodiacetic acid substrates in the production of N-(phosphonomethyl)glycine compounds. See generally, Franz, et al., Glyphosate: A Unique Global Herbicide (ACS Monograph 189, 1997) at pp. 233-62 (and references cited therein); Franz, U.S. Pat. No. 3,950,402; Hershman, U.S. Pat. No. 3,969,398; Felthouse, U.S. Pat. No. 4,582,650; Chou, U.S. Pat. Nos. 4,624,937 and 4,696,772; Ramon et al., U.S. Pat. No. 5,179,228; Ebner et al., U.S. Pat. No. 6,417,133 Leiber et al., U.S. Pat. No. 6,586,621 and Leiber, U.S. Pat. Nos. 6,927,304 and 6,956,005. The entire disclosure of the patents referred to in this paragraph and all other patents and publications referred to throughout this application are incorporated herein by reference.
High concentrations of formaldehyde in the reaction product solution resulting from oxidative cleavage of an N-(phosphonomethyl)iminodiacetic acid substrate are undesirable. The formaldehyde by-product is undesirable because it reacts with N-(phosphonomethyl)glycine to produce unwanted by-products, mainly N-methyl-N-(phosphonomethyl)glycine (NMG), which reduces the N-(phosphonomethyl)glycine yield. In addition, the formaldehyde by-product itself is undesirable because of its potential toxicity. See Smith, U.S. Pat. No. 5,606,107.
Franz, U.S. Pat. No. 3,950,402, discloses oxidizing the formaldehyde by-product to carbon dioxide and water simultaneously with the oxidative cleavage of the N-(phosphonomethyl)iminodiacetic acid substrate by using a heterogeneous oxidation catalyst comprising a noble metal deposited on a carbon support. The noble metal on carbon oxidation catalyst may be referred to as “bifunctional” in that the carbon component provides the primary adsorption site for the oxidation of the N-(phosphonomethyl)iminodiacetic acid substrate to form the N-(phosphonomethyl)glycine product and formaldehyde, while the noble metal component provides the primary adsorption site for the oxidation of formaldehyde and formic acid to form carbon dioxide and water, thus giving the following overall reaction:

The noble metal component may also tend to reduce the rate of deactivation of the catalyst (i.e., prolong the useful life of the catalyst). In addition to the N-(phosphonomethyl)glycine product, formaldehyde, formic acid and unreacted N-(phosphonomethyl)iminodiacetic acid substrate, the oxidation product solution may also contain other by-products, such as N-methyl-N-(phosphonomethyl)glycine (NMG), N-formyl-N-(phosphonomethyl)glycine (NFG), aminomethylphosphonic acid (AMPA), methyl aminomethylphosphonic acid (MAMPA), iminodiacetic acid (IDA), glycine, glyoxylic acid, phosphoric acid, phosphorous acid and imino-bis-(methylene)-bis-phosphonic acid (iminobis) and mixtures thereof.
Even though the Franz method produces an acceptable yield and purity of N-(phosphonomethyl)glycine, high losses of the costly noble metal by dissolution into the reaction mixture (i.e., leaching) typically result. Under the oxidation conditions of the reaction, some of the noble metal is oxidized into a more soluble form and organic components of the reaction solution, such as the N-(phosphonomethyl)iminodiacetic acid substrate and the N-(phosphonomethyl)glycine product, may act as ligands that solubilize the noble metal and/or sequester the solubilized noble metal in organic chelate complexes. After the N-(phosphonomethyl)glycine product has been formed and the noble metal catalyst has been separated from the reaction mixture, the oxidation product solution may be concentrated (e.g., by evaporation) to precipitate N-(phosphonomethyl)glycine product crystals and then separate the solid product from the various by-products and impurities retained in the resulting depleted reaction solution or mother liquor. Although a substantial quantity of the mother liquor may be recycled within the process, commercial considerations typically dictate that at least a portion of this residual reaction solution be purged from the system to avoid the build up of undesirable impurities and by-products that may compromise product purity. This purging unavoidably results in the loss of at least some of the solubilized noble metal and thereby undermines the economic feasibility of the process. Furthermore, the presence of solubilized noble metal within the reaction mixture typically results in incorporation of some noble metal into the N-(phosphonomethyl)glycine product resulting in additional loss of the noble metal.
Like Franz, Ramon et al., U.S. Pat. No. 5,179,228, teach using a noble metal deposited on the surface of a carbon support to catalyze the oxidative cleavage of an N-(phosphonomethyl)iminodiacetic acid substrate. To reduce the problem of noble metal leaching (reported to be as great as 30% noble metal loss per cycle) Ramon et al. teach flushing the reaction mixture with nitrogen under pressure after the oxidation reaction is completed to cause re-deposition of the noble metal onto the surface of the carbon support. According to Ramon et al., nitrogen flushing reduces the noble metal loss to less than 1%.
More recently, attention has focused on developing bifunctional noble metal on carbon oxidation catalysts that resist noble metal leaching (i.e., exhibit improved compositional stability) and provide increased activity and/or selectivity, particularly with respect to oxidation of formaldehyde into carbon dioxide and water (i.e., increased formaldehyde activity). Ebner et al., U.S. Pat. No. 6,417,133, disclose so-called “deeply reduced” noble metal on carbon catalysts for use in the oxidative cleavage of an N-(phosphonomethyl)iminodiacetic acid substrate and oxidation of other oxidizable reagents and methods for their preparation. Such deeply reduced catalysts exhibit remarkable resistance to noble metal leaching in aqueous, acidic oxidation reaction media. As a result, the catalyst disclosed by Ebner at al. provides for substantially quantitative oxidation of N-(phosphonomethyl)iminodiacetic acid substrates to N-(phosphonomethyl)glycine products, while minimizing noble metal losses and maintaining effective oxidation of the formaldehyde and formic acid by-products of the reaction for a prolonged period and/or over numerous oxidation cycles.
Although the teachings of Ebner et al. are significant and make economically practical the otherwise unavailable advantages provided by noble metal on carbon catalysts in the preparation of N-(phosphonomethyl)glycine products by oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid substrates, noble metal losses in aqueous waste streams purged from the process and noble metal losses in N-(phosphonomethyl)glycine product streams cannot be completely avoided and represent a significant operational cost. That is, despite the improvement in catalyst stability and general resistance to noble metal leaching provided by the deeply reduced catalyst of Ebner et al., overall process economics are still diminished to some extent by the leaching of noble metal under the severe acidic oxidation reaction conditions which include the presence of the N-(phosphonomethyl)glycine product and other organic components that may act as ligands and exacerbate noble metal leaching even from stabilized bifunctional catalyst systems. Accordingly, a need exists for effective techniques for recovering the solubilized noble metal from aqueous process streams produced in the preparation of N-(phosphonomethyl)glycine products by the noble metal-catalyzed oxidation of N-(phosphonomethyl)iminodiacetic acid substrates. The recovered noble metal could be reclaimed and advantageously used in the preparation of fresh catalyst to significantly improve overall economics of the process.